The major pathway of chloramphenicol (CAP, RNHCOCHCl2) metabolism in rat liver microsomes is oxidative dechlorination to CAP oxamic acid (RNHCO2H). We have now, however, observed at least 5 additional minor metabolites of CAP by HPLC analysis of incubation mixtures. Two of these metabolites have been identified by comparison of their HPLC retention time and mass spectra with authentic compounds. One of the compounds is p-nitrobenzyl alcohol (NO2C6H4CH2OH). This metabolite is only produced under aerobic incubation conditions. The second compound is monochloro-CAP (RNHCOCH2Cl). In contrast to the first identified metabolite, this product is only produced under conditions of low oxygen tension. The mechanism of formation of these metabolites and their potential bone marrow toxicity is currently being evaluated.